1. Field of the Invention
This invention relates to a new design for a cylindrical ionization detector.
2. Description of the Related Art
In a cylindrical ionization detector, sensing of the position of Compton and photoelectric gamma interactions is of great value in imaging applications. Moreover, measurement of the radial coordinate can be used to correct for the spatial dependence of the amount of charge sensed by the electrodes, which otherwise degrades energy resolution. While position sensing is common in proportional detectors like multiwire counters [G. Charpak and F. Sauli. Nucl. Instrum. Meth., 162:405, 1979; J. L. Lacy and R. S. Lindsey. High-resolution readout of multiwire proportional counters using the cathode-coupled delay-line technique. Nucl. Instrum. Meth., 119:483–498, 1974], where low gas pressures allow high signal gains, appropriate methods for ionization chambers, where signals are of low amplitude, have not been fully explored. A couple of studies have proposed position sensing based on electron drift time measurements, with limited success [N. G. Goleminov, B. U. Rodionov, and V. Yu. Chepel. Position sensitive xenon gamma-quantum detector. Instruments and Experimental Techniques, 29(2):325–328, 1986; G. Tepper and J. Losee. A compressed xenon ionization chamber X-ray/gamma-ray detector incorporating both charge and scintillation collection. Nuclear Instruments and Methods in Physics Research A, 368:862–864, 1996]. On the other hand, the geometrical dependence of energy resolution in cylindrical ionization detectors can be addressed by the use of Frisch grids. However, they are sensitive to microphonic noise, structurally difficult to implement, and degrade spectroscopic resolution by adding substantial capacitive load to the charge sensitive amplifier, thereby producing increased noise levels.